For aviation platforms, achieving the highest performance with the lightest weight systems that exceed the required reliability standards is paramount. Military aircraft often emphasize mission capability as the key objective, while commercial aircraft often emphasize Specific Fuel Consumption (SFC) and overall life-cycle costs. Historically, mechanical, hydraulic, and pneumatic drive systems have been used on these platforms for Environmental Control Systems (ECS), and actuation for engine and flight systems. With recent advances in high-speed bearing and cooling technologies, high-speed electrical machines are now able to achieve power densities that are competitive with or better than the aforementioned conventional drive systems. New systems are also increasingly required to have intelligent actuation control, such that they monitor their own health. All of these demands have led to a trend where the conventional mechanical, hydraulic, and pneumatic drive systems are being replaced with electrical systems. The increased use of such systems brings with it an increased need for thermal management systems.
In addition to the “electrification” trend of conventional components, demand for on-board power by conventional electronic systems has been increasing. Modern aircraft have increasingly powerful ECS, In Flight Entertainment (IFE), and avionics systems; all adding to the demand for onboard power. This trend in civilian aircraft has been preceded by a similar surge in the power requirements for military aircraft, where platforms are moving to the concept of the More Electric Aircraft (MEA). Increasingly powerful avionics, fly-by-wire, electronic warfare, and radar systems have resulted in huge increases in the demand for onboard power. Considering the smaller dimensions of these military aircraft platforms, the trend of onboard power demand on a per-passenger or per-volume basis is even greater for military than for civilian technology.
Although most electrical systems used onboard aircraft are designed to be highly efficient, the sheer magnitude of onboard power demand and unique design aspects of onboard aviation systems lead to considerable thermal management challenges. For example, even if a 1 MW system were 95% efficient, a total of 50 kW of heat would have to be rejected from the aircraft without exceeding rated temperatures. This poses significant challenges to the thermal management system, as it has to remove all of the generated waste heat with minimal temperature rise at a minimum of weight and volume.
Significant thermal management is required at all levels of power management, including generation, distribution, and conversion power electronics (PE), as well as at the application level. At the application level, heat is mostly produced by avionics, or, if present, actuators for flight control surfaces. Efficient thermal management is also required for digital electronics, power electronics and Environmental Control Systems (ECS). It is not uncommon for these systems to be cascaded; meaning that heat rejected from one system is added to the heat load of a secondary system before being rejected to an ultimate heat sink such as fuel or air.
In addition to the challenge of moving the heat efficiently to the convective surface, the ultimate rejection of heat is a challenge for aviation platforms. Airborne platforms have only two heat sinks available: fuel and ambient air. Fuel is a convenient sink for several reasons. A large quantity of fuel is available and must be carried on the aircraft regardless. Heating fuel prior to it entering the engine combustor is advantageous to the engine efficiency, although this is limited by the thermal stability of conventional jet fuel which, if compromised, can foul heat transfer surfaces. Therefore, it is generally preferred that thermal losses associated with electrification and any other unwanted heat sources are rejected to the fuel. There are exceptions, for instance, if a component sits in the area of an aircraft where the ambient air can effectively accept the losses without the need of a scoop or other component that increases aerodynamic drag. In that case, there would not be a need to route fuel to that area.
The More Electric Aircraft (MEA) concept has pushed the use of fuel as a heat sink to the limit. For some short missions on military aircraft, the amount of fuel that will be carried is determined by the electrical heat load rejection capacity requirements rather than the estimated engine fuel consumption.
While there is a large amount of fuel on-board an aircraft, its use is not evenly distributed over the flight envelope. During ground idle and idle-descent the fuel flow is very low. During take-off it is extremely high. Thus the fuel flow to the combustor nozzles rarely matches the electrical loss removal demands required by an MEA. For instance, an aircraft electrical system often requires significant cooling during idle-descent when electrical loads are relatively high (e.g., from actuation of flight control surfaces), but fuel flow is extremely low. To meet the cooling requirements, it is often required that the fuel is circulated back into the fuel tank after being used for cooling. This return-to-tank arrangement is common on military platforms. Using the fuel tank for thermal energy storage is convenient, but also has its limitations. At the beginning of the mission the hot fuel returning to tank does not significantly increase the overall temperature of the fuel in the tank because of the large thermal mass available. As the mission progresses and fuel levels are reduced, the high temperature return fuel has an increasingly greater likelihood of raising the temperature of the fuel in the tank.
The other ultimate heat sink is the ambient air. Air is abundantly available around the aircraft. The quality of this air for use as a heat sink varies widely. On the ground, air can be extremely cold or hot. The air density at 2,700 meters is about two-thirds that at sea level, and at 5,500 meters the air density is one-half that at sea level. Although air at high altitude is cold, viscous heating in the boundary layer between the air and the aircraft can be significant, especially at supersonic velocities. These factors combined make air a much less effective cooling fluid than fuel at high altitude, especially at high speeds.
Exchanging heat with air is also challenging. Air is not a great heat transfer fluid, thus significant heat exchanger surface area is often needed. During flight, air can be scooped from the surrounding aircraft space and passed through a duct where it interacts with various heat exchangers. Cooling air obtained in this manner is known as “ram air”. Because of the aircraft velocity, little or no fan power is needed to drive the air through the heat exchangers. However, scooping air changes the aerodynamics of the aircraft and induces drag, resulting in a fuel burn increase. In addition, if the components dumping heat into this heat sink require cooling during ground idle, a fan or similar fluid pumping device is required to move air across the heat exchangers, or the heat exchangers must be designed very large to allow for natural convection based cooling. In the engine area, heat exchangers can be placed in the bypass airstream in various ways, including a traditional heat exchanger directly in the airflow, or a surface cooler inside the engine fan bypass.
As the need to reject heat to the ambient increases with increasingly more electric aircraft, traditional solutions such as fuel cooling and ram air-cooling become problematic. With the transition to composite skin, fuel-efficient aircraft and increased power demands for aircraft subsystems such as on-board entertainment systems or advanced radars, significant thermal constraints have arisen wherein the ability of the aircraft to cool critical systems can be compromised during certain flight conditions. For any given aircraft with an established set of thermal constraints, the inability to sufficiently cool such critical systems can result in reduced flight envelopes, component failures, and even loss of aircraft. Analyzing pre-defined missions provides insight into potential thermal problems that may or may not develop during a mission. Although such insight is beneficial in determining mission capability for the aircraft, the information quickly becomes obsolete once the pilot and/or environmental characteristics deviate significantly from the assumed operating conditions originally analyzed. Therefore, missions where a pre-flight analysis indicates no thermal constraints may in fact result in reduced capability during the mission as a product of unanticipated operating or environmental conditions. In the same light, planned mission capability could possibly be extended if indications of current excess thermal capacity are observed in flight. There therefore exists a need for systems and methods to reliably predict aircraft thermal capacity during flight.